Prior art systems are known in which the existence of a single resonant circuit in a detection field or zone is utilized as an anti-theft type apparatus. Essentially, if an article having a single resonant frequency tag passes through a detection zone, an alarm is generated which indicates the unauthorized presence of store goods in the detection zone. Such resonant circuits have been constructed in accordance with standard printed circuit board techniques.
Some prior RF tagging systems have provided multiple different tuned (resonant) circuits on a tag so as to specifically identify the goods to which the tag is attached or the destination to which those goods should be directed. Such systems have been proposed for parcel or other article delivery systems wherein resonant circuits are utilized to provide a destination or sender code rather than printed bar codes.
The use of resonant circuit tagging is advantageous in that it is not subject to problems such as dirt obscuring a portion of a printed bar code and causing an error in determining the code associated with the article. Also, exact alignment of the tag with the detection system may not be required in RF tagging systems, since generally it is desired only to detect the presence of the resonant circuits somewhere in a broad detection zone. This can be achieved without precise alignment between the resonant circuit, the detection zone and the detection apparatus. However, prior systems utilizing multiple tuned circuit detection contemplate sequentially generating or gating each of the different resonant frequency signals to a transmitter antenna, and then waiting for reflected energy from each of the tuned circuits to be detected. Some frequency tagging systems look for absorption of RF energy by a resonant circuit during the transmission of each test frequency signal.
Generally, each different resonant frequency in a multiple frequency system is provided by a master oscillator circuit or transmitter whose output is essentially swept or stepped to sequentially provide each desired output frequency. In all of these systems the result is essentially a slow detection system since the systems sequentially radiate each of the different frequencies. Rapid detection is achieved only if there are a few different frequencies involved.
Some prior RF tagging systems contemplate printing a large number of different resonant frequency circuits on a tag and then creating different codes by the selective adjustment of some of these resonant circuits. These systems have recognized that it may be necessary to adjust the resonant frequency provided for each circuit and such adjustment is generally contemplated as occurring by selective removal of metalization forming the resonant circuit. Some systems have recognized that step adjustments of the resonant frequency of such tuned circuits is desirable and this has been implemented by punching holes of predetermined diameters in capacitive elements of the resonant circuit to thereby reduce capacitance and increase the frequency of the resonant circuit. Such known prior techniques are not readily adaptable to mass production of customized resonant frequency codes by a post factory manufacturing operation. Many times, the actual code to be utilized will not be known until immediately prior to attaching a tag or label to an article.
When it is possible to accurately control the orientation between the resonant multiple frequency tag and the detection zone, some prior systems have noted that fewer different resonant frequencies may be needed to produce the desired end coding result. However, these prior systems accomplish this result by just limiting the number of circuits in the detection zone so that the zone can only accommodate a few different tuned circuits at one time. This has the undesirable effect of effectively requiring wide spacing between tuned circuits on a tag and therefore undesirably increasing the size of the tag on which the tuned circuits are provided.
An improved RF tagging system is fully described in copending application Ser. No. 07/966,653, filed on Oct. 26, 1992, in the names of Sanjar Ghaem, Rudyard L. Istvan, and George L. Lauro, for RF Tagging System and RF Tags and Method, which application is assigned to the assignee of the present invention and fully incorporated herein by reference. The system there disclosed includes, as a significant feature, the simultaneous radiation of RF energy at a plurality of different frequencies in order to detect each of a plurality of different frequency resonant circuits which may be provided on a tag. Then a code signal indicative of which resonant frequencies for the tag resonant circuits were detected is provided. The above feature results in a much faster detection of which resonant frequency circuits are provided on a tag in a detection zone. The cross-referenced application further describes an advantageous configuration for step frequency adjusting the resonant frequencies of resonant circuits on a tag and additionally, an RF tagging system which utilizes focused narrow radiation beams for detection of individual resonant circuits on a multiple resonant frequency tag. Also, disclosed are preferred RF tag configurations/ constructions and a method of making such tags. Additionally, the aforementioned cross-referenced application describes RF tagging system features related to the use of phase shifting/polarization, object approach detection and measuring both voltage and current signals so as to provide improved RF tag detection systems.
In accordance with the present invention, it has been recognized that shifts in the resonant frequencies of multiple tuned resonant circuits can be caused by RF properties of the tagged items to which the resonant frequency circuits are in close proximity. The shifts in the resonant frequencies of the resonant circuits results from contents in the tagged items interacting with the resonant circuits on the RF tag. The magnitude in which resonant frequencies are shifted is a function of two mutually independent parameters: (1) frequency dependent distortions or shifts; and/or (2) spatially dependent distortions or shifts. In the case of frequency dependent distortions or shifts, the RF characteristics of the tagged item will vary with frequency. Interaction between the tagged item and the resonant frequency circuits on the tag will be more pronounced at certain frequencies than others. In the case of spatially dependent distortions or shifts, the proximity of the resonant frequency circuits to the RF disturbing elements in the tagged item effect the degree of the frequency shifts. Some resonant circuits will be closer to disturbing elements in the item than others and will thus experience more pronounced frequency shifts than other resonant circuits which are more distant from the RF disturbing elements in the tagged item.
The present invention provides an improved tagging system having resonant frequency shift compensation. In accordance with the present invention, the frequency dependent and/or spatial dependent components of the resonant frequency shifts are detected by determining the actual resonant frequencies of reference resonant circuits on a tag. Thereafter, the difference between the actual resonant frequencies of the reference resonant circuits and the undisturbed resonant frequencies of the reference resonant circuits is determined for each reference resonant circuit and compensation factors are provided for each data resonant circuit. Responsive to the compensation factors, the resonant frequency detector determines the resonant frequencies of the data resonant circuits for generating a code indicative of which data resonant circuits are on the tag. Hence, in accordance with the present invention, two sets of reference resonant circuits may be used. A first set of reference resonant circuits may be used for detecting spatially dependent resonant frequency shifts and a second set of reference resonant circuits may be used for detecting the frequency dependent resonant frequency shifts. The present invention has aspects applicable to manufacturing variation compensation and compensation for other disturbances such as temperature, mechanical deformation or other objects, besides the tagged object, which alter a designed resonant mode on a tag.